Attached hereto and forming a part hereof is a list of references. The references are enumerated and are referred to in the text by their numbers (1, 2, 3 . . . etc.).
Laser beams have been combined interferometrically, but to do so all output beams must maintain a fixed phase relationship. The complexity of such interferometric techniques has precluded their practical use.
Nonlinear, photo-refractive materials have also been suggested for laser beam combining, but they are limited to low intensity of illumination (a few watts per cm.sup.2).
Raman beam combining in molecular gases has also been suggested, but has drawbacks such as breakdown of the gas under intense laser fields and a large frequency shift (10.sup.13 Hz) due to the Raman effect. The present invention is believed to operate (although no limitation to any particular theory of operation is intended by this reference) by a stimulated Rayleigh scattering mechanism which imposes a much smaller frequency shift (3.times.10.sup.6 Hz) in the combined, intense output beam and is not as subject to breakdown in the medium in which transfer of energy (beam combining) occurs. Even then, the medium will heal, should breakdown occur.
It is known that that interaction of a single intense laser field with a two-level atom modifies the energy level structure of the atom. This modification is conveniently described in terms of "dressed" atomic states..sup.1 Transitions among the dressed states give rise to resonances in the atomic response, some of which can lead to amplification of a weak probe wave. Mollow.sup.2 has calculated the weak-probe-wave absorption spectrum of a strongly driven collection of atoms and predicts three spectral features, two of which can lead to amplification of the probe wave. One of these gain features occurs when the probe is detuned by the generalized Rabi frequency and is known as stimulated three-photon scattering. The other gain feature occurs when pump wave is detuned from the pump wave by the inverse of the lifetime of the excited state and is a form of stimulated Rayleigh scattering..sup.3-9 Wu et al..sup.10 observed this predicted absorption spectrum using a sodium atomic beam as the two-level system. More recently,.sup.11 the inventors hereof have shown that both the Rabi-sideband and Rayleigh resonances can give rise to large amplification of a weak probe wave in sodium vapor, where the atomic number density can be much higher than in an atomic beam. In particular, they reported 38- and 4-fold increases in the probe intensity at the Rabi-sideband then Rayleigh resonances, respectively, in a 7-mm interaction length. The inventors have discovered that high gain near the Rabi-sideband and Rayleigh resonances in the weak-probe wave is comparable to that of the strong pump wave, and efficient coupling interaction occurs in a two-level system in the presence of two strong optical fields.
The interaction of a two-level system with two strong fields has been considered in a number of previous investigation..sup.12-17 Gush and Gush.sup.12 and Tsukada.sup.13 used continued-fraction techniques to solve the density-matrix equations of motion for the interaction of two strong rf magnetic fields with a two-level system. Bonch-Breuvish et al..sup.14 presented a theory for the case of a pump field tuned exactly to resonance and equal pump and probe intensities which predicts resonances in the probe transmission whenever the probe-pump detuning is equal to a subharmonic of the Rabi frequency. In addition, they demonstrated that the interaction of strong rf magnetic fields with cadmium vapor agreed with the results of their theory. The existence of subharmonic resonances in the probe absorption spectrum are also predicted in the interaction of a two-level atom with two electromagnetic fields of arbitrary intensities and frequencies..sup.15,16 In addition, Tsukada and Nakayama.sup.15 showed that the Rayleigh resonance is present when the pump fields are tuned off-resonance. Agarwal and Nayak.sup.17 showed theoretically that the strength of the subharmonic resonances decreases rapidly as the laser bandwidth increases. Experimental studies of the interaction of two intense optical fields with a two-level system include that of Hillman et al.,.sup.18 and that of Chakmakjian et al.,.sup.19 who observed subharmonic resonances in the response of a sodium atomic beam driven by a 100%-amplitude-modulated exciting field.